UPDATE: If you've come to us through Google, you'll be delighted to hear that we just launched our dedicated iPhone as an e-book reader forum!
Join the discussions here --Alex, 07/24/08
To my surprise, the iPhone has turned out to finally fill the mobile PDF needs I have tried to satisfy with the Sony eBook reader and my TabletPC. The PDF reader built-in to the system is fast and supports zoom and landscape view, making it very easy to read any document, especially if it has two or more columns, which fit the screen really well. You either need to store the PDFs as email attachments, save them to something like box.net to view using the browser, or else jailbreak your phone and use the PDFViewer software, but these are relatively easy...